Spatial distribution and diurnal variation of cumuliform clouds during Indian Summer Monsoon

The Indian Summer Monsoon brings most of the annual rainfall to India and its neighborhood. Since cumuliform clouds contribute the major fraction of rain, it is important to study the spatial and diurnal distribution of the probability of occurrence of cumuliform clouds (POC). For this study, Meteosat-5 infrared data for June to September of 2003 to 2006 are used. A wavelet-based method to extract all cumuliform clouds is used. The method uses the a trous wavelet transform (ATWT) to separate small-scale (<40 km) structures in brightness temperature images. The monthly POC distribution shows large areas of high POC on the west coast of India (June and September), Arakan coast in Bay of Bengal (all months), northeast India (all months), and east coast of India (September). Over land, POC peaks in the evening (1600 to 1800 Indian Standard Time (IST)) while over the oceans its peak time is in the morning (similar to 0800 IST). The land-sea contrast in POC along the coast during evening time is remarkable. Over the ocean, POC varies sinusoidally, but over land it has a narrow peak with flat minimum. The cloudiness minimum over land lasts longer (similar to 10 h) than that over the oceans (<5 h). South-facing slopes of the Himalayas, Khasi hills, and isolated smaller land regions show a diurnal cycle of cumuliform clouds opposite to that of the diurnal cycle of land regions, while afternoon maximum is observed over the middle Himalayan ranges and Tibetan plateau. POC for brightness temperature (BT) < 210 K are improved with finer resolution. Cold clouds are found to be less prominent over Western Ghats and middle Himalayan ranges. Hovmoller plots show that cumuliform clouds travel across the domain from east to west, while mesoscale features originating over the Western Ghats in the peninsula move west to east from evening to morning. Such propagation of convective systems is so regular that it appears even in 4-year-average POC data of wavelet transform and BT threshold of 220 K.